Electrical apparatuses using batteries as an electric power supply have been widely used. In electrical apparatuses of this kind, output can be increased and operation time can be extended by using a plurality of batteries. Thus, where a plurality of batteries connected in series is used, the output of the electrical apparatus can be increased, and where a plurality of batteries connected in parallel is used, the operation time of the electrical apparatus can be extended.
Japanese Patent Application Publication No. 2000-308268 discloses an electric power tool using two battery packs as an electric power supply. In this electric power tool, the attached two battery packs can be connected in series and also can be connected in parallel. Therefore, when the electric power tool is used for a heavy operation, the output of the electric power tool can be increased by connecting the two battery packs in series, and when the electric power tool is used for a light operation, the operation time of the electric power tool can be extended by connecting the two battery packs in parallel.